1. Field of the Invention
The present invention relates to a multi-component system for modifying, degrading or bleaching lignin, lignin-containing materials or similar substances and to processes for its use.
2. The Prior Art
The sulfate process and the sulfite process are mentioned as the processes currently used chiefly for pulp production. With both processes, pulp is produced by cooking and under pressure. The sulfate process operates with the addition of NaOH and Na.sub.2 S, while Ca(HSO.sub.3).sub.2 +SO.sub.2 is used in the sulfite process.
All the processes have as the primary objective, removing of the lignin from the plant material, wood or annual plants used.
The lignin which, with the cellulose and the hemicellulose, makes up the main constituent of the plant material (stem or trunk) must be removed. Otherwise, it is not possible to produce papers which are non-yellowing and which can be subjected to high mechanical stress.
Wood pulp production processes operate with stone grinders (mechanical wood pulp) or with refiners (TMP), which defibrillate the wood by grinding after appropriate pretreatment (chemical, thermal or chemical-thermal).
These wood pulps still comprise most of the lignin. They are used primarily for the production of newspapers, illustrated journals and the like.
The possibilities of the use of enzymes for degradation of lignin have been researched for some years. The action mechanism of such lignolytic systems was clarified only a few years ago. Then it became possible to obtain adequate amounts of enzyme with the white rot fungus Phanerochaete chrysosporium under suitable growing conditions with additions of inductor. The previously unknown lignin peroxidases and manganese peroxidases were discovered by this research. Since Phanerochaete chrysosporium is a very effective degrader of lignin, attempts were made to isolate its enzymes and to use them in a suitable form for lignin degradation. However, this was not successful, since it was found that the enzymes lead above all to repolymerization of the lignin and not to degradation thereof.
Similar circumstances also apply to other lignolytic enzyme species, such as laccases, which degrade the lignin oxidatively with the aid of oxygen instead of hydrogen peroxide. It was found that similar processes occur in all cases. In fact, free radicals are formed which react with one another again and thus lead to polymerization.
There are thus currently only processes which operate with in vivo systems (fungus systems). The main key points of optimization experiments are so-called biopulping and biobleaching.
Biopulping is understood as meaning treatment of chopped wood chips with live fungus systems. There are 2 types of forms of application:
1. Pretreatment of chopped chips before refining or grinding in order to save energy during the production of wood pulps (for example TMP or mechanical wood pulp). One advantage is the improvement which usually exists in the mechanical properties of the pulp, but a disadvantage is the poorer final whiteness.
2. Pretreatment of chopped chips (softwood/hardwood) before cooking of the pulp (kraft process, sulfite process).
The objective is reduction in cooking chemicals, improvement in cooking capacity and extended cooking. Improved kappa reduction after cooking in comparison with cooking without pretreatment is also achieved as an advantage.
Disadvantages of these processes are clearly the long treatment times (several weeks), and above all the unsolved risk of contamination during treatment if sterilization of the chopped chips, which is uneconomical, is to be dispensed with.
Biobleaching likewise operates with in vivo systems. The cooked pulp (softwood/hardwood) is seeded with fungus before bleaching and is treated for days to weeks. Only after this long treatment time is a significant reduction in kappa number and increase in whiteness found. This renders the process uneconomical for implementation in the usual bleaching sequences.
Another application carried out usually with immobilized fungus systems is the treatment of waste waters from the manufacture of pulp, in particular bleaching waste waters. This treatment is for decolorization thereof and reduction of the AOX (reduction of chlorinated compounds in the waste water caused by chlorine or chlorine dioxide bleaching stages).
It is furthermore known to employ hemicellulases and also xylanases and mannanases as bleaching boosters.
These enzymes are said to act chiefly against the xylan which is reprecipitated after the cooking process and partly masks the residual lignin. Degradation thereof increases the accessibility of the lignin to the bleaching chemicals (above all chlorine dioxide) used in the subsequent bleaching sequences. The savings in bleaching chemicals demonstrated in the laboratory were confirmed to only a limited extent on a large scale. Thus, this type of enzyme can at best be classified as a bleaching additive.
Chelating substances (siderophors, such as ammonium oxalate) and biosurfactants are assumed to be a cofactor, alongside the lignolytic enzymes.
The Application PCT/EP87/00635 describes a system for removing lignin from material containing lignin-cellulose with simultaneous bleaching. This system operates with lignolytic enzymes from white rot fungi with the addition of reducing and oxidizing agents and phenolic compounds as mediators.
In DE 4,008,893 C2, mimic substances which simulate the active center (prosthetic group) of lignolytic enzymes are added in addition to the redox system. It was thus possible to achieve a considerable improvement in performance.
In the Application PCT/EP92/01086, a redox cascade with the aid of phenolic or non-phenolic aromatics coordinated in oxidation potential is employed as an additional improvement.
The limitation for use on a large industrial scale is the applicability at low pulp densities (up to not more than 4%) for all three processes. For the last two Applications the risk of leaching out of metals when using chelating compounds, can lead above all to destruction of the peroxide in the subsequent peroxide bleaching stages.
Processes in which the activity of peroxidase is promoted by means of so-called enhancer substances are known from the three publications WO 94/12619, WO 94/12620 and WO 94/12621.
The enhancer substances are characterized with the aid of their half-life in WO 94/12619.
According to WO 94/12620, enhancer substances are characterized by the formula A.dbd.N--N.dbd.B, in which A and B are each defined cyclic radicals.
According to WO 94/12620, enhancer substances are organic chemicals which contain at least two aromatic rings, at least of which is substituted in each case by defined radicals.
All three publications relate to dye transfer inhibition and to the use of the particular enhancer substances, together with peroxidases, as a detergent additive or detergent composition in the detergent sector. A possible use for the treatment of lignin is referred to in the description of these WO Applications. However, the Applicants' own experiments with the substances disclosed completely in these publications have shown that they showed no activity as mediators. Thus, they did not increase the bleaching action of the peroxidases during treatment of lignin-containing materials.
WO 94/29510 describes a process for enzymatic delignification in which enzymes are employed together with mediators. Compounds having the structure NO--, NOH-- or HRNOH are generally disclosed as mediators.
Of the mediators disclosed in WO 94/29510, 1-hydroxy-1H-benzotriazole (HBT) gives the best results in the delignification. However, HBT has various disadvantages: It is available only at high prices and not in adequate amounts.
It reacts under delignification conditions to give 1H-benzotriazole. This compound is relatively poorly degradable, and can represent considerable environmental pollution in larger quantities. It leads to damage to enzymes to a certain extent. Its rate of delignification is not at all that high.